It is of interest in many cases to know the speed of a vehicle. The vehicle driver needs the speed data to gain some indication of the expected reactions of his vehicle during certain driving maneuvers. It should naturally also be noted that he must adhere to specific speed restrictions. The most precise knowledge possible of the current speed of his own vehicle is also desirable in order to estimate a possible risk during an overtaking action. The speeds of other vehicles or other moving bodies can also be deduced from knowledge of the speed.
The speed of a vehicle, e.g. a motor vehicle on a road, is usually determined from the rotational speed of a vehicle axle. So-called speedometer drives are known, which by mechanical, and more recently electronic, conversion calculate the number of distance units covered, in particular kilometers, as well as the vehicle speed from the rotational speed of the vehicle front axle or of elements connected thereto, and indicate these to the vehicle driver. These measurements are precise within limits and their accuracy of indication is also prescribed to increase road safety.
The conventional methods for measuring vehicle speed generally work from the rotational movement of the vehicle axle. Therefore, they measure the rotational speed of the vehicle axle relative to the vehicle chassis or another non-rotating part of the vehicle. While the measurement is more precise in motor vehicles, there still remain inaccuracies, since the measurement of the rotational speed of the vehicle axle and the subsequent evaluation for display in the motor vehicle must be based on fixed values of the wheel diameter and naturally must disregard any slip between the vehicle axle and the wheel, on the one hand, and between the wheel and the road surface, on the other.
It is proposed for a bicycle in document DE 100 37 377 A1 to clamp electromagnetic or electrostatic or similar sensors to the front wheel of the vehicle itself, so that a corresponding bicycle speedometer can be detached and attached as easily as possible. A calculating counter should also be attached together with the sensor to the spokes of the bicycle. No further details are presented regarding the type of measurement.
A sensor system is evident from document DE 10 2005 014 500 A1, with which vibrations at the wheel are to be detected to determine any anomalies in wheel movement.
Most of the measuring devices used have the disadvantage that they are difficult to mount, because concealed parts of the vehicle have to be accessed. More recently, so-called bus systems are also being used in vehicles to transmit information such as vehicle speed, for example, in the vehicle. Normally no measuring devices may be connected to these for safety-related reasons. Moreover, adaptation is generally necessary for different types of vehicle. In addition, the measuring device must be parameterised and calibrated for each vehicle, since the methods do not supply absolute measured values, but only work with values proportional to speed such as the number of wheel revolutions per minute, for example.
It is also known in principle to obtain the speed from the integration of an acceleration. This method is well suited to deduce the speed v(t) from a known speed v0 according to the following formula:v(t)=v0+∫a(t)dt 
However, in this case the speed v0 must be adjusted again and again, since the advancing integration leads to a drift in the measured value and therefore after only a short period precise values are no longer delivered.
Higher requirements for accuracy are set for a measurement of speed of a vehicle, for example, where measurements in traffic control are concerned. Thus, the speed of a vehicle travelling in front can also be determined from a travelling vehicle travelling behind it, wherein a particularly precise knowledge of one's own speed is necessary to also keep relative measurements usable and conclusive.
This is also problematic because the speed conventionally displayed on the speedometer of the following vehicle must also be picked up or secured in some suitable form to that it can be related to the observed data for the preceding vehicle. Such a pick-up is technically difficult, since there should naturally be no intervention into the safety regions of the following vehicles.
A combination of two gravimetric rotational sensors, which record the accelerations in the vicinity of the rotary axis of vehicle wheels and determine the vehicle speed therefrom, is known from U.S. Pat. No. 6,466,887 B1.
EP 0 517 082 A2 describes a method and a device for tamper-free detection of the wheel speed of vehicles by monitoring the number of revolutions of the wheels. In this case, acceleration magnitudes are determined that correspond to the acceleration due to gravity and/or centrifugal acceleration.
Nevertheless, there remains the desire for more precise possibilities of measuring the speed of vehicles. Moreover, there is the desire to also measure other vehicle data and, for example, determine details about the diameter of a wheel automatically during travel.
Therefore, it is an object of the present invention to propose a measuring device and a method for measuring vehicle data, which in particular allow as precise a measurement as possible of the speed of a vehicle running on wheels and a determination of the diameter of the vehicle wheel during travel.